Aircraft: EMB ERJ 170/175 ER/LR
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy

During cruise at FL360; I began to notice a small line of dark brown discoloration on my windshield (F/O side windshield); which had originated from the bottom left corner and was lengthening towards the right side; half-way up the windshield. I pointed this out to the Captain immediately; and at first glance; it looked like some sort of fluid was streaming up the windshield; but we noticed that as the line was lengthening; it was also widening and darkening. At that point; I realized it was most likely a failure of the Windshield 2 Heater; and that it was overheating and de-laminating. I asked the Captain if he agreed that it might be a windshield overheat; at which point he said it is a definite possibility; and to be ready with the QRH procedure for a Windshield 2 Overheat caution EICAS message. Sure enough; a few seconds later; the EICAS message 'WINDSHIELD 2 HTR FAIL' posted; and I started turning to the appropriate procedure in the QRH. Within 10 seconds of the EICAS message; we heard a loud popping sound; and I looked up to find that my windscreen had completely cracked; to the point of appearing shattered; and it appeared that the heating element was melting the glass. At this point; I called 'Oxygen; don masks'; and both the Captain and I donned our masks. There were no indications of cabin pressurization issues; but I had chosen to don my mask and call for both of us to don our masks in case the inner pane of safety glass had been compromised. I looked as closely as I could at the glass to see if the inner pane had been compromised; but I wasn't able to discern the extent of the damage to my windshield. As such; my prerogative was to take the most conservative action possible and don masks; as well as keeping my head down below the glare shield in case the glass failed. We kept our masks on; and I kept my head down for the duration of the flight. After the glass shattered and we had donned our masks; the Captain assumed control of the aircraft and ATC communications; and called for the Cracked Windshield QRH procedure. Simultaneously; he asked ATC for a lower altitude; to which ATC responded that we could descend to FL270. We began a normal descent; and the Captain regulated our airspeed to less than 250 KIAS. I began to run the procedure; which involves turning off the windshield heat. Since the windshield cracked before we could run the Windshield 2 HTR FAIL QRH procedure; this is the point where the windshield heat button was pushed out. Next; the QRH calls for an evaluation of the condition of the windshield. At this point; both the Captain and I examined the windshield as closely as possible; but neither of us was able to determine if the inner layer had been compromised. It was clear to both of us that both the outer and middle layers were compromised; but we could not discern the condition of the inner pane. Next; the QRH calls for a descent to 10;000 feet or MEA; whichever is higher. The Captain then requested a descent to 10;000 feet and declared an emergency with ATC. The QRH notes that the descent shall be accomplished in 5 minutes or less; and I had mentioned this to the Captain. We had been descending at roughly 1;500 FPM; and I mentioned that we needed to descend quicker. At this point; he set the speed to 250 KIAS; selected FLCH mode; and commanded full flight spoilers. Next; the QRH calls for selecting the pressurization mode selector to MAN; and selecting Cabin Altitude to increase. As outlined in the QRH; I selected the Pressurization mode selector to MAN; which caused the cabin altitude to increase at a rate of about 2;000 FPM. Recalling that the cabin altitude knob is very sensitive; I momentarily moved it to the UP position in order to modulate cabin pressure to a higher altitude. While this was happening; the Captain had asked me to take ATC communications while he contacted the flight attendants and asked them to take their seats. While talking with ATC; I had noticed that the cabin climb rate was very high; at one point exceeding 5;000 FPM. The Captain re-assumed control of ATC communications and I continued with the QRH procedure. Due to task saturation between the emergency descent; ATC communication and cabin communication; I was not able to maintain cabin altitude to 10;000 feet. During the communication swap; the cabin altitude exceeded 12;400 feet; causing the oxygen masks in the cabin to drop. Passing through about FL200; the Captain asked ATC for direct ZZZ; and was cleared as requested. About 30 seconds after the Captain requested direct ZZZ; I had noticed that ZZZ1 airport was closer to our present position and recommended that we change our diversion to ZZZ1. He agreed and we requested direct ZZZ1; and were cleared as requested. Once we arrived at 10;000 feet MSL; we evaluated the situation; again examined the windshield; and determined that it appeared to be in the same condition as when we began our emergency descent. ZZZ was only an additional 10 minutes away; so we decided that ZZZ would be a more suitable diversion for a multitude of reasons. So we requested to precede direct ZZZ. Once we were direct ZZZ; the Captain asked if I would call the flight attendants and check their condition; as well as that of the cabin and passengers. They notified me that they were in good shape; the passengers were fine; and that the Captain had briefed them already on the TEST items. I verified with them that it would be a normal landing; and deplaning would take place at the gate; and would be normal deplaning procedures. Next; the Captain and I briefed the arrival and visual approach items. We continued with a normal visual approach; and the Captain landed the aircraft safely. During taxi; the Captain asked me to again brief the flight attendants that when we arrived at the gate; we would leave the fasten belts sign on and keep the passengers seated until the Captain could de-brief them about what had happened. I made a PA to the passengers as we taxied and asked them to stay in their seats once we arrived at the gate; at which time the Captain would de-brief them; and thanked them for their patience. Once we arrived at the gate; we shut down the engines; completed the parking checklist; and the Captain proceeded to de-brief the passengers. After the de- brief was complete; the passengers de-planed normally. I performed a post-flight inspection of the aircraft and found no other issues or damage. The windshield overheating was the result of a defective Windshield Heating unit. This could not have been predicted or prevented by the flight crew or maintenance. Also; I understand that a windshield heating unit failure will likely lead to a cracked windshield because the fail-safe logic in the unit forces it to shut off automatically; causing a rapid change in windshield temperature which leads to the crack. The primary suggestion that I have in this incident is in relation to the QRH procedure for an Impaired or Cracked Windshield. It calls for Pressurization Mode Selector - MAN and Cabin Altitude INCREASE; which I accomplished per the QRH. In doing so; the cabin altitude increased at a rapidly increasing rate. Our cabin altitude went from 8;000 feet to over 12;500 feet in approximately one minute. To attempt to modulate cabin altitude during a high workload event such as an emergency descent; especially while communicating with cabin crew and ATC makes it easy to miss the note below asking the pilot to maintain a cabin altitude of 10;000 feet. In my opinion; this procedure should be re-written in one of two ways: 1) During Descent: Pressurization Mode Selector.....MAN; Cabin Altitude.....Maintain 10;000 feet; or 2) During Descent: Pressurization DUMP Button.....Press. In scenario 1; the instructions are explicit from the beginning that the objective is not simply to open the outflow valve; but rather to maintain a specific cabin altitude. The note as it is currently written makes it seem like maintaining a 10;000 feet cabin altitude is not a priority; or that it is secondary to selecting manual pressurization and cabin altitude increase. In scenario 2; rather than selecting the cabin pressurization mode to MAN; the pilot would leave it in AUTO and simply push the DUMP button. Per system logic; this automatically modulates the outflow valve to maintain a 2;000 FPM cabin rate of climb; which is much more controlled than what we had experienced even after just placing the Pressurization Mode Selector to MAN. Had the rate of climb in the cabin been controlled automatically; it would have reduced our workload during the emergency descent; and we likely would have been able to keep the masks from deploying; since the aircraft rate of descent was much greater than 2;000 FPM. This probably would have prevented the passenger oxygen masks from deploying. EBM170 First Officer describes a windshield heat failure that results in a cracked windshield and diversion to a suitable airport.